Synergistically optimizing electrical and thermal transport properties of Bi2O2Se ceramics by Te‐substitution

Bi₂O₂Se oxyselenides, characterized with intrinsically low lattice thermal conductivity and large Seebeck coefficient, are potential n‐type thermoelectric material in the mediate temperature range. Given the low carrier concentration of ~10¹⁵ cm^?3 at 300 K, the intrinsically low electrical conductivity actually hinders further enhancement of their thermoelectric performance. In this work, the isovalent Te‐substitution of Se plays an effective role in narrowing the band gap, which notably increases the carrier concentration to ~10¹⁸ cm^?3 at 300 K and the electron conduction activation energy has been lowered significantly from 0.33 to 0.14 eV. As a consequence, the power factor has been improved from 104 μW·K^?2·m^?1 for pristine Bi₂O₂Se to 297 μW·K^?2·m^?1 for Bi₂O₂Se_0.96Te_0.04 at 823 K. Meanwhile, the suppressed lattice thermal conductivity derives from the introduced point defects by heavier Te atoms. The gradually decreased phonon mean free path reflects the increasingly intense phonon scattering. Ultimately, the ZT value attains 0.28 for Bi₂O₂Se_0.96Te_0.04 at 823 K, an enhancement by a factor of ~2 as compared to that of pristine Bi₂O₂Se. This study has demonstrated that Te‐substitution of Se could synergistically optimize the electrical and thermal properties thus effectively enhancing the thermoelectric performance of Bi₂O₂Se.     

Substrate temperature-dependent thermoelectric figure of merit of nanocrystalline Bi2Te3 and Bi2Te2.7Se0.3 prepared using pulsed laser deposition supported by DFT study

This work reports the pulsed laser deposition of n-type selenium (Se) doped bismuth telluride (Bi₂Te_2.7Se_0.3) and n-type bismuth telluride (Bi₂Te₃) nanostructures under varying substrate temperatures. The influence of the substrate temperature during deposition on the structural, morphological and thermoelectric properties for each phase was investigated. Density functional theory (DFT) simulations were employed to study the electronic structures of the unit-cells of the compounds as well as their corresponding partial and total densities of states. Surface and structural characterization results revealed highly crystalline nanostructures with abundant grain boundaries. Systematic comparative analysis to determine the effect of Se inclusion into the Bi₂Te₃ matrix on the thermoelectric properties is highlighted. The dependence of the thermoelectric figure of merit (ZT) of the nanostructures on the substrate temperatures during deposition was demonstrated. The remarkable room temperature thermoelectric power factor (PF) of 2765 μW/mK² and 3179 μW/mK² for pure and Se-doped Bi₂Te₃ compounds respectively, signifies their potential of being useful in cooling and power generation purposes. The room temperature ZT values of the Se-doped Bi₂Te₃ was found to be 0.92, about 30% enhancement as compared with the pure phase, which evidently results from the suppressed thermal conductivity in the doped species caused by phonon scattering at the interfaces.     

Boosting the thermoelectric performance of Bi2O2Se by isovalent doping

N‐type Bi₂O₂Se has a bright prospect for mid‐temperature thermoelectric applications on account of the intrinsically low thermal conductivity. However, the low carrier concentration of Bi₂O₂Se (~10¹⁵ cm^?3) severely limits its thermoelectric performance. Herein, the boosting of the carrier concentration to ~10¹⁹ cm^?3 can be realized in our La‐doped Bi₂O₂Se ceramic samples, which could be ascribed to the formation of isoelectronic traps and the narrowing of band gap, and contribute to a marked increase in the electrical conductivity (from 0.03 S cm^?1 to 182 S cm^?1). Our X‐ray absorption near‐edge structure spectra results reveal that a local disordering of oxygen atoms could be an important reason for the intrinsically low thermal conductivity of Bi₂O₂Se, and the point defects can also suppress the lattice thermal conductivity in La‐doped Bi₂O₂Se. The ZT value can be enhanced by a factor of ~4.5 to 0.35 at 823 K for Bi_1.98La_0.02O₂Se as compared to the pristine Bi₂O₂Se. The coordinated optimization of electrical and thermal properties demonstrates an effective method for the rational design of high‐performance thermoelectric materials.     

Europium (III) doped bismuth telluride decorated on carbon-based materials for enhancing thermoelectric performance

Scalable synthetic approach for superior performance of thermoelectric (TE) materials is a crucial step for the TE technology progress. Herein, reduced graphene oxide (RGO), carbon nitride (g-C ₃N₄) and europium (Eu) are utilized as additives to bismuth telluride (Bi₂Te₃) matrix to prepare various novel nanocomposites (NCs): (RGO@Bi_1.8Te₃Eu_0.2) and (RGO-g-C₃N₄@Bi_1.8Te₃Eu_0.2) with an enhanced TE performance. The novel NCs were synthesized via solvothermal method, physiochemically characterized and consolidated into pellets of 1 mm thickness to measure their TE properties. The new additives potentially affected the physicochemical and TE properties of Bi₂Te₃. Nanostructured hexagonal nanoplatelets with 12.5 nm thickness were observed by scanning and transmission electron microscopy (SEM and TEM) of the synthesized Bi₂Te₃. This thickness shrinked to 5.7 and 5.2 nm upon the formation of (RGO@Bi_1.8Te₃Eu_0.2) and (RGO-g-C₃N₄@Bi_1.8Te₃Eu_0.2) NCs, respectively. Energy dispersive X-ray Spectroscopy (EDS) of NCs proved the existence of Bi, Te, C, Eu and N atoms. Raman and Fourier-transform infrared (FT-IR) spectra confirmed the NC formation that led to narrowing the energy band gap of Bi₂Te₃ as displayed by UV–Vis spectra. Brunauer–Emmett–Teller showed specific surface area expansion of Bi₂Te₃ from 6.78 to 19.00 and 16.75 m²g^-1 of (RGO@Bi_1.8Te₃Eu_0.2) and (RGO-g-C₃N₄@Bi_1.8Te₃Eu_0.2) NCs, respectively. The electrical conductivity of Bi₂Te₃ rose by 56 and 69 times, whereas its thermal conductivity significantly dropped by 1.6 and 1.7 times upon (RGO@Bi_1.8Te₃Eu_0.2) and (RGO-g-C₃N₄@Bi_1.8Te₃Eu_0.2) NCs formation. Owing to extra channels of carrier transfer and phonon scattering induced by NCs heterointerfaces. Novel combination of carbon-based materials and Eu with Bi₂Te₃ matrix boosts its TE performance resulting in a worthy candidate for power generation applications at room-temperature.     

Thermoelectric properties of Cu-dispersed bi<Subscript>0.5</Subscript>sb<Subscript>1.5</Subscript>te<Subscript>3</Subscript>

A novel and simple approach was used to disperse Cu nanoparticles uniformly in the Bi_0.5Sb_1.5Te₃ matrix, and the thermoelectric properties were evaluated for the Cu-dispersed Bi_0.5Sb_1.5Te₃. Polycrystalline Bi_0.5Sb_1.5Te₃ powder prepared by encapsulated melting and grinding was dry-mixed with Cu(OAc)₂ powder. After Cu(OAc)₂ decomposition, the Cu-dispersed Bi_0.5Sb_1.5Te₃ was hot-pressed. Cu nanoparticles were well-dispersed in the Bi_0.5Sb_1.5Te₃ matrix and acted as effective phonon scattering centers. The electrical conductivity increased systematically with increasing level of Cu nanoparticle dispersion. All specimens had a positive Seebeck coefficient, which confirmed that the electrical charge was transported mainly by holes. The thermoelectric figure of merit was enhanced remarkably over a wide temperature range of 323-523 K.     

Fabrication and electrical properties of Bi2-xSbxTe3 ternary nanopillars array films

Highly oriented Bi_2-xSb_xTe₃ (x?=?0, 0.7, 1.1, 1.5, 2) ternary nanocrystalline films were fabricated using vacuum thermal evaporation method. Microstructures and morphologies indicate that Bi_2-xSb_xTe₃ films have pure rhombohedral phase with well-ordered nanopillars array. Bi, Sb and Te atoms uniformly distributed throughtout films with no precipitation. Electrical conductivity of Bi_2-xSb_xTe₃ films transforms from n-type to p-type when x?>?1.1. Metal-insulator transition was observed due to the incorporation of Sb in Bi₂Te₃. Bi_2-xSb_xTe₃ film with x?=?1.5 exhibits optimized electrical properties with maximum electrical conductivity σ of 2.95?×?10⁵ S?m^?1 at T?=?300?K, which is approximately ten times higher than that of the undoped Bi₂Te₃ film, and three times higher than previous report for Bi_0.5Sb_1.5Te₃ films and bulk materials. The maximum power factor PF of Bi_0.5Sb_1.5Te₃ nanopillars array film is about 3.83?μW?cm^?1 K^?2 at T?=?475?K. Highly oriented (Bi,Sb)₂Te₃ nanocrystalline films with tuned electronic transport properties have potentials in thermoelectric devices.     

Enhanced thermoelectric performance in three-dimensional superlattice of topological insulator thin films

We show that certain three-dimensional (3D) superlattice nanostructure based on Bi₂Te₃ topological insulator thin films has better thermoelectric performance than two-dimensional (2D) thin films. The 3D superlattice shows a predicted peak value of ZT of approximately 6 for gapped surface states at room temperature and retains a high figure of merit ZT of approximately 2.5 for gapless surface states. In contrast, 2D thin films with gapless surface states show no advantage over bulk Bi₂Te₃. The enhancement of the thermoelectric performance originates from a combination of the reduction of lattice thermal conductivity by phonon-interface scattering, the high mobility of the topologically protected surface states, the enhancement of Seebeck coefficient, and the reduction of electron thermal conductivity by energy filtering. Our study shows that the nanostructure design of topological insulators provides a possible new way of ZT enhancement.     

The influence of CNTs on the thermoelectric properties of a CNT/Bi2Te3 composite

CNT/Bi₂Te₃ composites were prepared from composite powders in which CNTs were implanted in the Bi₂Te₃ matrix powders by a novel chemical route. It was found that the fabricated composite had a microstructure of a homogeneous dispersion of CNTs in the Bi₂Te₃ matrix due to interfacial bonding agents of oxygen atoms attaching to the surface of CNTs. The dimensionless figure of merit (ZT) of the composite shows significantly increased values compared to those of pure binary Bi₂Te₃ in the temperature range of 298–498 K and a maximum ZT of 0.85 was obtained at 473 K. It is considered that the improved thermoelectric performance of the composite mainly originated from thermal conductivity that was reduced by active phonon-scattering at the CNT/Bi₂Te₃ interface.     

Enhancements of thermoelectric performance in n-type Bi2Te3-based nanocomposites through incorporating 2D Mxenes

Bi₂Te_2.7Se_0.3 compound has been considered as an efficient n-type room-temperature thermoelectric (TE) material. However, the large-scale applications for low-quality energy harvesting were limited due to its low energy-conversion efficiency. We demonstrate that TE performance of Bi₂Te_2.7Se_0.3 system is optimized by 2D Ti₃C₂T_x additive. Here, a 43% reduction of electrical resistivity is obtained for the nanocomposites at 380 K, originating from the increased carrier concentration. Consequently, the g = 0.1 sample shows a maximum power factor of 1.49 Wmm^?1K^?2. Meanwhile, the lattice thermal conductivity for nanocomposite samples is reduced from 0.77 to 0.41 Wm^?1K^?1 at 380 K, due to the enhanced phonon scattering induced by the interfaces between Ti₃C₂T_x nanosheets and Bi₂Te_2.7Se_0.3 matrix. Therefore, a peak ZT of 0.68 is achieved at 380 K for Bi₂Te_2.7Se_0.3/0.1 wt% Ti₃C₂T_x, which is enhanced by 48% compared with pristine sample. This work provides a new route for optimizing TE performance of Bi₂Te_2.7Se_0.3 materials.     

Preparation and thermoelectric properties of MoS2/Bi2Te3 nanocomposites

MoS₂ nanosheets with size of several-hundred nanometers were prepared by a hydrothermal intercalation/exfoliation method, then MoS₂/Bi₂Te₃ composite nanopowders were prepared by a microwave-assisted wet chemical method using the MoS₂ nanosheets, TeO₂, Bi(NO₃)₃·5H₂O, KOH and ethylene glycol as raw materials. Bulk MoS₂/Bi₂Te₃ nanocomposites were prepared by hot pressing the MoS₂/Bi₂Te₃ composite nanopowders with MoS₂ nanosheet content ranging from 0 to 17 wt% at 80 MPa and 648 K in vacuum. X-ray photoelectron spectroscopy and X-ray diffraction analyses indicate that MoS₂ and Bi₂Te₃ did not react each other during the hot pressing. FESEM observation reveals that the MoS₂/Bi₂Te₃ composite samples had a more compact microstructure than the pristine Bi₂Te₃ bulk sample. The MoS₂ phase was relatively randomly dispersed in the composite. At a given temperature, the electrical conductivity of the composites increases first then decreases as the MoS₂ content increases, whereas the Seebeck coefficient of the bulk nanocomposites does not change much. A highest power factor, ~18.3 μW cm⁻¹ K⁻² which is about 30% higher than that of pristine Bi₂Te₃ sample, at 319 K has been achieved from a nanocomposite sample containing 6 wt% MoS₂.     

High thermoelectric performance achieved in Bi0.4Sb1.6Te3 films with high (00l) orientation via magnetron sputtering

To obtain p-type Bi–Sb–Te-based thin films with excellent thermoelectric performance, the Bi_0.4Sb_1.6Te₃ target is prepared by combining mechanical alloying with the spark plasma sintering technique. Afterward, Bi_0.4Sb_1.6Te₃ thin films are deposited via magnetron sputtering at variable working pressures. With an increasing working pressure, the frequency of collisions between the argon ions and sputtered atoms gradually increases, the preferred orientation of (00l) increases, and the sputtering rate decreases. The Seebeck coefficient increases from ∼140 μV/K to ∼220 μV/K as the carrier concentration decreases along with an increasing working pressure. Furthermore, the decrease in carrier concentration and acceleration of carrier mobility also affect the change in electrical conductivity. The maximum power factor of the p-type Bi_0.4Sb_1.6Te₃ thin film deposited at 4.0 Pa and at room temperature exceeds 20.0 μW/cm K² and is higher than that of most p-type Bi–Sb–Te-based films.     

Effects of mineral fillers addition and preparation method on the morphology and electrical conductivity of epoxy/multiwalled carbon nanotube nanocomposites

This study investigated the correlation between the electrical conductivity and the micro and nanomorphology of multiwalled carbon nanotubes (MWCNTs)/epoxy nanocomposites with and without the inorganic fillers montmorillonite (MMT), sepiolite and calcium carbonate (CaCO₃). The nanocomposites were prepared by dispersing the MWCNT and fillers through ultrasonication directly in the resin or solvent. For nanocomposites without fillers, the compositions prepared with solvent demonstrated higher electrical conductivities, which correlate with a microscale morphology formed by networks of highly interconnected MWCNT agglomerates. The addition of MMT induced a deleterious effect on the electrical conductivity of the nanocomposites since this filler hinders the formation of MWCNT agglomerate networks. The effect of sepiolite on electrical conductivity is also negative, but in this case, nonmorphological effects are likely of greater importance. The addition of CaCO₃ improved the electrical conductivity of the binary nanocomposites under specific conditions. For this filler, a synergic effect was achieved for the composition prepared with solvent, which resulted in an approximately sixfold increase in electrical conductivity relative to the nanocomposite without filler.     

Thermoelectric properties of the textured Bi1.9Gd0.1Te3 compounds spark-plasma-sintered at various temperatures

Elemental composition, crystal and grain structures, specific electrical resistivity, Seebeck coefficient, thermal conductivity, and thermoelectric figure-of-merit of n-type grained Bi_1.9Gd_0.1Te₃ compounds, spark-plasma-sintered at T_S = 690, 720, 750, 780 and 810 K, have been studied. All the samples are highly textured along the 001 direction parallel to the pressing direction. The average grain size measured along the pressing direction is much less as compared to the average grain size measured in the perpendicular direction. A strong anisotropy in the transport properties measured along directions parallel and perpendicular to the pressing direction was found within the 290 ÷ 630 K interval. Electrical resistivity decreases and thermal conductivity increases for parallel orientation as compared to these properties for perpendicular orientation. The T_S - effect on thermoelectric figure-of-merit of textured Bi_1.9Gd_0.1Te₃ compounds has been found and analyzed. Highest thermoelectric figure-of-merit (∼0.75) was observed for sample with T_S = 750 K at perpendicular orientation.     

Hydrothermal method for the synthesis of Sb2Te3, and Bi0.5Sb1.5Te3 nanoplates and their thermoelectric properties

In this study, a modified hydrothermal method is reported for the preparation of Sb₂Te₃ and Bi_0.5Sb_1.5Te₃ nanoplates and their bulk samples was prepared by spark plasma sintering (SPS). The crystal structure, morphology, and thermoelectric properties were investigated. The microstructure results indicate that the bulk samples consisted nanograins after SPS. The presence of nanograins, high Seebeck coefficient (181 μV/K), high electrical conductivity (763 Ω^?1 cm^?1), and low thermal conductivity (1.15 W/mK) has been achieved in Sb₂Te₃ nanoplate bulk samples. As a result, the dimensionless thermoelectric figure of merit (ZT) of 0.55 at 400 K was achieved. Moreover, the peak ZT shifted to higher temperature compared with other reported results found in literature.     

Comparative study of gamma and ion beam irradiation of polymeric nanocomposite on electrical conductivity

Poly(vinyl alcohol) (PVA) was used to prepare nanocomposites of multi‐wall carbon nanotubes (MWCNT) and functionalized carbon nanotubes (MWCNT‐NH₂) in existence of 2‐carboxyethyl acrylate oligomers (CEA). Radiation‐induced crosslinking of the prepared matrix was carried out via gamma and ion beam irradiation. A comparative study of gamma and ion beam irradiation effect on the electrical conductivity of nanocomposite was conducted. The gelation of the gamma irradiated matrix outperforms the ion beam irradiated matrix. The order of gelation is PVA > (PVA/CEA) > (PVA/CEA)‐MWCNT > (PVA/CEA)‐MWCNT‐NH₂. There is a significant reduction in the swelling of the nanocomposite. The formation of nanocomposites was confirmed by scanning electron microscopy, energy‐dispersive X‐ray (EDX) and FTIR examinations. The direct current electrical properties of PVA/nanocomposites are examined at room temperature by applying electric voltage from 1 to 20 V. The results revealed that the electrical conductivity is increased by adding the carbon nanotubes and irradiation by gamma and ion beam. At an applied electric voltage 20 V, in the electrical conductivity of the unirradiated PVA was from 9.20 × 10^?8 S cm^?1. After adding MWCNT an increase up to 4.70 × 10^?5 S cm^?1 was observed. While after ion beam irradiation, a further increase up to 9.30 × 10^?5 S cm^?1 was noticed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46146.     

A simple energy-saving aqueous synthesis of Bi2Te3 nanocomposites yielding relatively high thermoelectric power factors

We discover a simple scalable (10?g scale for one batch in this study) route of synthesizing Bi₂Te₃ nanocomposites in aqueous solution with high yield at room temperature without involving any organic chemicals, capping agents, and surfactants. It is conceivable that the formation mechanism involves interaction between elemental Bi and Te, which takes place at a very slow rate and takes about 2 weeks to form Bi₂Te₃. Heat treatment of Bi₂Te₃ nanocomposite yields a single phase of Bi₂Te₃ with the relatively high power factor of 24.2?μW/cm?K² at 425?K compared to other solution methods.     

Thermal‐Insulation,Electrical, and Mechanical Properties of Highly‐Expanded PMMA/MWCNT Nanocomposite Foams Fabricated by Supercritical CO2 Foaming

Foaming behavior of poly(methyl methacrylate) (PMMA)/multi‐walled carbon nanotubes (MWCNTs) nanocomposites and thermally‐insulating, electrical, and mechanical properties of the nanocomposite foams are investigated. PMMA/MWCNT nanocomposites containing various amounts of MWCNTs are first prepared by combining solution and melt blending methods, and then foamed using CO₂. The foaming temperature and MWCNT content are varied for regulating the structure of PMMA/MWCNT nanocomposite foams. The electrical conductivity measurement results show that MWCNTs have little effect on the electrical conductivity of foams with large expansion ratio. Thermal conductivities of both solid and foamed PMMA/MWCNT nanocomposites are measured to evaluate their thermally insulating properties. The gas conduction, solid conduction, and thermal radiation of the foams are calculated for clarifying the effects of cellular structure and MWCNT content on thermal insulation properties. The result demonstrates that MWCNTs endowed foams with enhanced thermal insulation performance by blocking thermal radiation. Moreover, the compressive testing shows that MWCNTs improve the compressive strength and rigidity of foams. This research is essential for optimizing environmentally friendly thermal insulation nanocomposite foams with enhanced thermal‐insulation and compressive mechanical properties.     

Improved thermoelectric properties of n-type Bi2S3 via grain boundaries and in-situ nanoprecipitates

To improve the thermoelectric properties of n-type Bi₂S₃ materials, a certain amount of SbCl₃ were added into Bi₂S₃ materials by a conventional melting method combined with plasma activated sintering (PAS) process. The Bi₂S₃-based materials evolve from the lamellar- to particle-like structures after SbCl₃ doping. The phonon scattering has strong enhancement through the increased grain boundaries and in-situ Bi₂S₃ nanoprecipitates, resulting in the low lattice thermal conductivity. Meanwhile, the high power factor is achieved because of the marked increase in the electrical conductivity. Hence, the synergistic effect of antimony and chlorine substitutions not only contribute to reduce the thermal conductivity but also tune the electrical transport properties, yielding a peak ZT value of ∼ 0.65 at 773 K for the Bi₂S₃-1%SbCl₃ sample.     

Orientation-controlled synthesis and characterization of Bi2Te3 nanofilms, and nanowires via electrochemical co-deposition

An electrochemical deposition technique based on co-deposition was used to deposit preferentially oriented Bi₂Te₃ nanostructures (nanofilm, and nanowire). The shared underpotential deposition (UPD) potentials for both Bi and Te co-deposition were determined by cyclic voltammetric measurements. The scanning probe microscopy (scanning tunneling microscopy (STM) and atomic force microscopy (AFM)) and the X-ray diffraction (XRD) data indicated that the electrodeposition of Bi₂Te₃ results in nanofilm-structured deposits with a preferential orientation at (0 1 5) and nanowired-structured deposits with a preferential orientation at (1 1 0) in acidic and basic (in the presence of ethylenediaminetetraacetic acid (EDTA)) medium, respectively. The results show that the nucleation and growth mechanism follows 3D mode in acidic solutions and 2D mode in basic solution containing EDTA additive. The optical characterization performed by reflection absorption Fourier transform infrared (RA-FTIR) spectroscopy showed that the band gap energy of Bi₂Te₃ nanostructures depends on the thickness, size, and shape of the nanostructures and the band gap increases as the deposition time decreases. Moreover, the quantum confinement is strengthened in the wire-like deposits relative to the film-like deposits. Energy dispersive X-ray spectroscopy (EDS) analysis demonstrated that Bi₂Te₃ nanostructures were always in 2:3 stoichiometry, and they were made up of only pure Bi and Te.     

Enhancing the thermoelectric properties of powder metallurgically produced BiSbTe alloy through feasible electroless surface coating method

The powder metallurgical techniques capable of powder mass production but lacks in achieving high zT. Therefore, to improve zT, electroless surface coating was performed on water atomized Bi_0.5Sb_1.5Te₃ (BST) powder and stimulated their conducting nature. Surface chemical composition studies displays fine elemental coating on the BST powders and remarkably, the Seebeck coefficient increased till 450 K. The surface coated bulk sample shows greatly improved power factor about ~45% higher than uncoated sample. Indeed, at high temperature we obtained reduced lattice thermal conductivity which confirms the remarkable suppression of bipolar thermal conductivity. Consequently, a maximum zT of 1.16 at 400 K achieved by coated samples which is ~102% enhancement compared to original BST. Also, we attained high average zT of ~0.99 which helps in employing thermoelectric devices across wide temperature. Therefore, electroless surface coating could be a feasible method for improving the properties of mass-produced BST powder.